Nickel-chromium and nickel-chromium-cobalt base alloys containing titanium and aluminum develop, on suitable heat-treatment, a high level of creep-rupture strength at high temperatures and are widely used in applications giving rise to high stress at elevated temperatures, such as gas turbine engine rotor blades and vanes. However, the need to use impure fuels such as diesel oil in land-based and marine propulsion turbines gives rise to sulfidation attack. Operation in marine and other chloride-containing environments also results in severe corrosion problems.
Many gas turbine and other components, particularly those of complex design, are best produced by precision casting, and there is thus a need for an alloy that can be cast to shape and possesses, in the cast form, a high level of strength at elevated temperatures in conjunction with good resistance to corrosion in sulfur- and chloride-containing environments and structural stability, i.e., freedom from sigma-phase formation, after extended service at elevated temperatures.
In U.S. Pat. No. 4,039,330 are described alloys that exhibit this combination of properties and contain from 0.02 to 0.25% carbon, from 20 to 25% chromum, from 5 to 25% cobalt, one or both of molybdenum (up to 3.5%) and tungsten (up to 5%) in such amounts that the value of %W+0.5 (%Mo) is from 0.5 to 5%, from 1.7 to 5% titanium and from 1 to 4% aluminum, with the provisos that the sum of the aluminum and titanium contents is from 4 to 7% and the ratio of titanium to aluminum is from 0.75:1 to 4:1, from 0.5 to 3tantalum, from 0 to 3% niobium, from 0.005 to 1.0% zirconium and from 0 to 1.99% hafnium, with the proviso that the value of %Zr+0.5 (%Hf) is from 0.01 to 1%, from 0.001 to 0.05% boron, and from 0 to 0.2% in total of yttrium or lanthanum or both, the balance, apart from impurities, being nickel in an amount of at least 30%. All the percentages and ratios in this composition range and elsewhere in the present specification, are by weight.
One alloy according to this specification is available commercially under the designation IN-939, with the nominal composition:
C 0.15; Cr 22.5%; Co 19%; W 2%; Ti 3.7%; Al 1.9%; Ta 1.4%; Nb 1.0%; Zr 0.1%; B 0.01%; Ni balance.
After heat-treatment consisting of solution-heating for 4 hours at 1150.degree. C., air-cooling and then aging for 16 hours at 850.degree. C., equiaxed castings of alloy IN-939 (made by vacuum melting followed by remelting and casting under vacuum) typically have a creep-rupture life at 870.degree. C. under a stress of 185N/mm.sup.2 (19 kgf/mm.sup.2) of about 1250 hours, which corresponds to about 850 hours at the same temperature under the higher stress of 200N/mm.sup.2. When the alloys are directionally-solidified to produce a columnar crystal structure the creep-rupture life, when stressed along the major crystal axis, is increased to about 1370 hours at 870.degree. C. and 200N/mm.sup.2.
In U.S. Pat. No. 4,039,330 creep-rupture test results are also given for two alloy compositions with and without additions of hafnium. Comparison of the results for the hafnium-containing and hafnium-free alloys showed that the presence of 0.75% hafnium had little or no effect on the creep-rupture life, though it produced some increase in the elongation at rupture.